A magnetic recording technique has a number of excellent characteristics which are not provided by other recording systems, such as the ability to repeatedly use the recording medium, ease of conversion of input data to an electronic signal, the ability to combine the recording technique with relevant equipment to form a system, and the ability to readily process a signal. Accordingly, magnetic recording techniques have been extensively applied to various fields such as the video, audio and computer fields. In order to meet the demand for miniaturization of equipment, enhancement of a recorded and reproduced signal, the trend to longer recording, and an increase in recording capacity, further improvement in recording density is needed. For a coating type magnetic recording disk, various means have been proposed in which the particle size of a ferromagnetic powder is decreased, the dispersibility thereof is improved and the packing density in a magnetic layer is increased. As further effective means, a ferromagnetic metal powder and hexagonal ferrite having excellent electromagnetic characteristics have been used.
The widespread use of OA equipment (i.e., Office Automation Equipment) such as a minicomputer and a personal computer has been accompanied by a marked increase in the popularity and use of a magnetic recording disk. A magnetic recording disk is used and stored under broad environmental conditions of temperature and humidity and in dusty environments. In particular, an improvement in recording density is strongly desired in order to achieve a large data recording capacity and miniaturization of the recording disk. In order to obtain a magnetic recording disk suitable for high density recording with an acicular ferromagnetic powder as in the past, it was necessary to employ a maximum size of the acicular ferromagentic powder sufficiently smaller than the recording wavelength or a record bit length. At present, an acicular ferromagnetic powder having a size of 0.3 .mu.m has already been brought into practical use, which makes it possible to record at wavelengths of 1 .mu.m or less.
It is necessary to further reduce the size of the acicular ferromagnetic powder in order to obtain a medium which enables even higher density recording. However, such a small-sized acicular ferromagnetic powder is disadvantageous in that the thickness thereof, which is 100 .ANG. or less, is very fine and the particle volume thereof, which is about 10.sup.-17 cm.sup.3, is very small. Consequently, the electromagnetic characteristics thereof are reduced by thermal disturbance and surface effects so that sufficient orientation cannot be obtained by applying a magnetic field to a coated magnetic layer.
In recent years, a high recording density magnetic recording medium has been developed based on a hexagonal series ferrite grain which is tabular and has an axis of easy magnetization perpendicular to a plate as a ferromagnetic powder as described, for example, in U.S. Pat. No. 4,425,401 corresponding to JP-A-58-6525 (the term "JP-A" as used herein means an unexamined published Japanese patent application) and JP-A-58-6526. This ferromagnetic powder allows for an average particle size of 0.05 .mu.m and high density recording.
Furthermore, a narrow track width is required for high density recording. In order to satisfy these requirements, the development and application in a magnetic disk of a ferromagnetic metal powder and a ferromagnetic hexagonal series ferrite for size miniaturization and improvement in recording density have been intensively investigated. In particular, the shift towards a thinner magnetic layer and high output are desired for achieving a high recording density and an improved overwriting electro-magnetic characteristic. However, there is a concern that the shift towards a thinner magnetic layer might be accompanied by a extreme deterioration of running durability.
An overwrite of the recording signals with different magnetic wavelengths is usually necessary in a magnetic recording disk for a computer such as a floppy disk. It was sufficient in the past to carry out the overwrite of two kinds of signals 1f and 2f which are in a relationship of two times in terms of frequency. However, not only a shorter recording wavelength but also the overwrite of plural RLL signals with a frequency ratio of 3:8 present at a broader range, are required for a magnetic recording disk with a high capacity of 10M bites, in which high capacity has been strongly desired in recent years. Where a signal having a short recording wavelength and a large difference in recording frequency is used, the improvement in electro-magnetic characteristics of a magnetic layer was the only limiting factor for successfully overwriting a signal with a short recording wavelength on a signal with a long recording wavelength as disclosed in U.S. Pat. No. 4,788,092 corresponding to JP-A-58-122623 and U.S. Pat. No. 4,895,758 corresponding to JP-A-61-74137.
In a conventional magnetic layer with a thickness of 1.0 .mu.m or more, overwrite of a signal with a shorter wave-length on a formerly recorded signal with a longer wavelength cannot erase the formerly recorded signal since a line of magnetic force cannot reach through the entire depth of the magnetic layer.
Furthermore, an improvement in recording density is accompanied by a narrower gap between the recording heads, which causes difficulty in sufficiently recording in the thickness direction of medium.
In order to solve the above problem, a thin magnetic layer of 1 .mu.m or less was proposed. However, the thin magnetic layer was liable to peel off and good running durability (a main factor for preventing dropout) could not be obtained, thus deteriorating reliability.
Accordingly, in developing a magnetic recording disk having the desired high density recording, improvement in reproducing output, securing an overwriting characteristic and running durability, in particular, have become obstacles.
Electrification in running of a magnetic recording disk increases the number of dropouts attributable to adherence of dust and the error rate thereby has become a fatal defect. In order to solve this electrification problem, methods are employed in which an additive is added in order to prevent electrification in a magnetic layer. Among them, the method in which carbon black is added is the most effective and broadly applied. However, in the above magnetic recording disk for high density recording, the addition of carbon black lowers the packing degree of a magnetic substance to deteriorate the output. Therefore, the addition amount of the carbon black is limited, so that the antistatic effect is insufficient.
In particular, the above described ferromagnetic hexagonal series ferrite powder has a low saturation magnetization as compared with those of a Co--Fe.sub.2 O.sub.3 ferromagnetic powder and a ferromagnetic metal powder. Consequently, it is difficult to obtain high output, and therefore the packing density of the ferromagnetic hexagonal series ferrite powder has to be increased in order to provide a magnetic recording disk with high output. However, since the hexagonal series ferrite powder is of a fine particle and is in the form of hexagon, the dispersibility thereof is inferior as compared with that of a conventional ferromagnetic powder. Thus, it is substantially difficult to obtain both good antistatic properties and high reproducing output.
Various proposals for preventing electrification and providing high output and improved durability are disclosed in JP-A-55-55431, JP-A-55-55432, JP-A-55-55433, JP-A-55-55434, JP-A-60-164926, JP-A-55-55436, JP-A-62-38523, and JP-A-62-159337.
Namely, an intermediate layer is provided between a magnetic layer and a support, wherein the intermediate layer containing carbon black and a binder resin is coated and then the magnetic layer is formed thereon.
Although effective for improving running durability, the electromagnetic characteristic of this method, namely, high reproducing output and an overwriting characteristic, is inadequate for high density recording in a magnetic recording disk.
The following additional difficulties have been encountered with conventional coating techniques when an extremely thin magnetic layer is coated, for example 0.5 .mu.m or less, in order to improve the overwriting characteristic:
(1) It is difficult to directly carry out coating on a non-magnetic support in an even thickness and the coated magnetic layer tends to peel off, and PA1 (2) The coating can be carried out on an intermediate layer (a non-magnetic layer) provided as a lower layer, but subsequent coating causes a loss of adhesion which in turn results in dropout due to peeling of the magnetic layer. PA1 1. A method in which a non-magnetic layer is first coated with a gravure coating, roll coating, blade coating or extrusion coating equipment and then while in a wet condition, a magnetic layer is coated with the non-magnetic support pressure type extrusion coating equipment disclosed, for example, in JP-B-1-46186 (the term "JP-B" as used herewith means an examined Japanese patent publication), and JP-A-60-238179 and JP-A-2-265672; PA1 2. A method in which a non-magnetic layer coating solution and a magnetic layer coating solution are simultaneously coated with a head having two built-in slits for extruding a coating solution as disclosed in JP-A-63-88080, JP-A-2-17971 and JP-A-2-265672; and PA1 3. A method in which a non-magnetic layer coating solution and a magnetic layer coating solution are almost simultaneously coated with extrusion coating equipment having a backup roll, as disclosed in JP-A-2-174965.
The investigations resulted in the finding that application of the wet-on-wet coating method (described in U.S. Pat. No. 4,844,946) in which a non-magnetic layer and a magnetic layer are simultaneously coated while they are in a wet condition is effective for solving the above problems.
However, application of this wet-on-wet coating method above could not sufficiently solve the following additional problem.
When a non-magnetic layer is provided, selection of a dispersing solvent for the magnetic layer is important. If the same kind of solvent is used for both the magnetic layer and the non-magnetic layer, the solvent used in coating the magnetic layer dissolves the non-magnetic layer when the magnetic layer is coated on the non-magnetic layer. The boundary between the non-magnetic layer and the magnetic layer is disturbed, which results in considerable variation in thickness of the magnetic layer. On the other hand, use of a solvent which does not dissolve the non-magnetic layer for coating the magnetic layer limits the above binder resins for use in the magnetic layer to a large extent. Additionally, coating with such solvents markedly deteriorates the adhesive force between the non-magnetic layer and the magnetic layer to thereby considerably reduce durability.
Namely, a non-magnetic layer coating solution and a magnetic layer coating solution are coated on a non-magnetic support in a wet condition. Therefore, the boundary between the non-magnetic layer and magnetic layer is disturbed, so that the thickness of the magnetic layer tends to vary. The generation of a portion at which the magnetic layer becomes thicker deteriorates the overwrite characteristic at that portion. Consequently, a previously recorded signal cannot be removed, so that there is a concern as to the ability to carry out reproduction. On the other hand, efforts to control the thickness variation in the magnetic layer make it difficult to obtain good durability.
Accordingly, efforts to minimize the thickness of the magnetic layer with conventional coating techniques to improve the overwrting characteristic results in increased thickness variation. This in turn makes it difficult to obtain a magnetic recording disk having a thin magnetic layer which has good overwriting characteristics over all tracks suitable for digital recording and high reproduction output, and which excels in running durability.
An effective method for solving these problems has not hitherto been proposed.